Technologically useful forms of strained silicon are those in which the silicon is strained by pseudomorphic growth as an ultra-thin layer of silicon on a silicon-germanium alloy buffer layer, or in which the strained silicon thus grown has been transferred as a thin layer onto an insulating layer on a second wafer so-called strained silicon on insulator, or else where the strain is induced by means of adjacent regions of silicon-germanium alloy.
These forms of strained silicon present difficulties to X-ray diffractometry or Raman spectroscopy when applied to them to measure the strain in the silicon layer. X-rays are not easily absorbed in silicon or in silicon-germanium alloy or insulators. Therefore, the depth from which information is generated in an X-ray diffractometry measurement is much larger than the technologically useful thickness of a strained silicon layer, typically 20 nm. When X-ray diffractometry is performed, for example, on a pseudomorphically grown ultra-thin layer of silicon on a silicon-germanium alloy buffer layer, almost all of the signal is generated from the underlying silicon-germanium alloy, and the crystallographic information determined is from this layer, even at grazing incidence of the x-ray and when measuring for long times. In extreme cases of measurement time and grazing incidence, information specific to the top strained silicon layer can be gleaned from an X-ray diffractometry measurement, but the time taken renders the measurement impractical for the examination of large numbers of samples in a short time. More generally, what is done is to measure the lattice constant of the underlying silicon-germanium alloy and calculate the strain in the top layer. This again is time-consuming, fails to measure the actual strain in the silicon itself, and is impossible in silicon-on-insulator structures.
The invention is directed towards achieving improved inspection of strained silicon or like materials.